This invention relates to a light-weight, thin-walled plastic container, in particular made of polyethylene terephthalate (PET), intended for post-filling pressurization.
In the packaging industry, in particular that of beverages, high-volume, thin-walled plastic containers present certain challenges. Such thin-walled containers are known in, for example, patent applications WO-03/033361, EP-1468930 and EP-1527999. Such containers are very attractive for small volumes (for example, and without limitation, containers of less than about 2 liters), due to the fact that the containers taught by these patents are relatively heavy because the amount of material is linked to the parameter of the volume of these containers.
The increase of the surface area of a package is not proportional to that of the volume of said package. Therefore, for large volume containers (for example, and without limitation, containers of more than about 2 liters), it is entirely possible to reduce the weight of the packages while ensuring good mechanical stability after packaging.
Also, such known containers exhibit the necessity for a pressurization of the inner volume that is not satisfactory as explained below.
In addition, for economic reasons that are easily understood, the object is to reduce the amount of material that is necessary for the production of these thin-walled containers, as much for reducing the production costs as for reducing the recycling costs, which the prior art does not allow since as soon as the volumes become large enough (e.g. greater than 2 liters, in particular), the weight of the material increases very significantly since it is linked to the volume, and is therefore a cubic factor.
These containers are disposable, and it would be advantageous to avoid unnecessary sophistication for the use to which they are put, hence the increased importance of the thin-walled containers but with an identical manufacturing process that allows a range going from small containers of several centiliters up to high volumes of one to several tens of liters by avoiding an excessive increase of the weight of the material.
In addition to the problem of reducing the amount of material, it is possible to mention another problem, that of rigidity, which proportionally decreases with the amount of material.
For these containers that are manufactured by the process according to the prior art with a reduced amount of material, the rigidity of the container that is obtained is inadequate. This rigidity is inadequate for allowing good gripping before opening, and primarily this low rigidity makes difficult, and even impossible, a superposition of these full containers, in particular when they are palletized and the pallets are stacked on one another.
In addition, the rigidity of such a thin-walled container poses another problem because these containers are packaged at ambient temperature and when these containers are placed in a cold environment, a collapsing phenomenon occurs that produces deformations of the container and poor stability during gripping.
In general, such thin-walled containers may be filled under cold conditions with flat liquids such as mineral water, oil, fruit juices, or milk. Then, to meet the requirement of rigidity, it is provided to put these thin-walled containers under internal pressure. For example, such a process may include the so-called “nitrogen drop” process that is currently used industrially, or any other analogous process. This nitrogen drop process consists in introducing a drop of liquid nitrogen into the filled container immediately before the head space of the container is sealed. Immediately after sealing, this drop of liquid nitrogen is transformed into gas. The increase in volume in the head space leads to a rise in pressure in the interior of the container and therefore to a rigidification of said container. This increase in pressure nevertheless remains relatively low (e.g. on the order of one-tenth of a bar).
However, this process of applying nitrogen poses a certain number of problems. Firstly, the metering of the volume that is introduced is difficult, since the final pressure depends on the amount that is introduced, working conditions, and the length of time of sealing. Secondly, the distribution means of this drop of nitrogen should be integrated in the production line, and, as a result, they should therefore be adapted. For example, when the packaging is produced aseptically, this adaptation is a high stress one, requiring cleaning, sterilization, and further maintenance. In production lines, an additional station involves an additional source of potential failure, which can shut down the entire production line. In the case of an aseptic packaging line, this intervention proves still more difficult because the interventions are difficult and time-consuming since it is necessary to restore the unit to aseptic packaging conditions.
In addition, it is noted that liquid nitrogen, at a greatly negative temperature, drops in the liquid to ambient temperature although the fall of the drop uniformly causes splashing on the edges of the container. These splashes of the contained fluid, such as mineral water, fruit juice, and oil, can degrade after packaging and/or during storage, leading to the development of mold before the product is marketed and therefore before the product is consumed, which is not satisfactory.
The material that is used for manufacturing the thin-walled containers is often PET, known for its transparency, low weight, and great shaping possibilities. PET also allows good preservation of contained liquids.